1. Field of the Invention
The present invention relates to an improved method of and apparatus for accurately measuring and indicating and/or recording the individual and cumulative lengths of open-ended tubes such as drill pipes and providing a signal for inventory control.
2. Description of the Prior Art
It has long been known in the prior art that acoustical or sound waves move in bounded media and that when boundaries are encountered reflection occurs. Evidence of the reflection of sound waves from a wall or barrier is the echo that is heard.
Reflection of sound waves also occurs when sound passing down a tube or pipe meets an abrupt change in area. This penomena has been the subject of much investigation and has been employed for many purposes including the determination of the location of openings and obstructions in tubes and the measurement of the lengths of pipes.
The historical background relating to the study of sound indicates that velocity in the air was the earliest acoustical quantity to be measured. In 1862, Regnault measured the velocity of sound in pipes for various conditions of temperature and humidity. See "ACOUSTICS," Alexander Wood, Blackie and Son Limited, London (1940), page 250.
In 1896, Lord Rayleigh calculated the correction in the length of an open pipe due to the effective or impedance reflective surface being spaced away from the end of the pipe. The correction was calculated to be 0.82 times the radius of the pipe for a pipe ending with an infinite flange. Experimentally, he found the end correction of an unflanged open pipe to be 0.6 multiplied by the pipe radius. See "The Theory of Sound" by Rayleigh, Dover Publications, New York (1896), page 487.
Helmholtz, in 1897, determined the correction in the length of an open pipe required to compensate for radiation loss and impedance change for a pipe without a flange at being 0.6 R, R being the pipe internal diameter divided by two. Helmholtz also determined the intensity of a reflected acoustic wave where a change in the pipe internal diameter occurs. See "TECHNICAL ASPECTS OF SOUND," edited by E. G. Richardson, Elsevier Publishing Company, New York (1953), pages 493 et seq.
A mathematical explanation of the reflection that occurs when sound passing down a tube meets an abrupt change in area is given on pages 9-38, 9-39, 9-44 and 9-45 in the HANDBOOK OF ENGINEERING FUNDAMENTALS comprising Volume 1 of the Wiley Engineering Handbook Series, published in 1936, under the Editorship of Ovid W. Eshbach by John Wiley & Sons, Inc., New York.
In 1962, Louden presented a paper at the FOURTH INTERNATIONAL CONGRESS ON ACOUSTICS in Copenhagen on a new method for determining the end correction of pipes by means of pulses. Until that time such measurements of the velocity of sound had been made using a continuous wave to excite a tube into resonance. Louden showed the relationship between "a", the correction factor, and the diameter to be about 0.65 for an open pipe without a flange.
Summarizing the foregoing with respect to the utilization of sound in determining the length of pipes, it has been known in the prior art that:
1. In an open pipe, a sound pulse with a wave length greater than the pipe internal diameter will propagate the length of the pipe and reflect from the open end the pipe.
2. The reflection occurs not at the exact end of the pipe but at a distance of 0.65 R beyond the end where R is the pipe radius. The spacing of the impedance reflective surface from the end of the pipe is thus a function of internal pipe diameter.
3. The velocity of sound in all but small pipes is that for sound in open air for a given temperature and humidity.
It has been proposed in the prior art, in U.S. Pat. No. 4,241,430, granted on Dec. 23, 1980 to D. J. Kayem et al, to determine the individual and cumulative length of numerous "joints" or lengths of drill pipe or casing by the use of a hand-held probe and a separate housing that is in electrical communication with the probe by a conduit or cable. The probe includes a switch, means to measure the ambient temperature, means to produce sound pulses, and means to detect acoustic waves created by reflected sound pulses. Provided for use with the probe are flanged tubes having the same length but different sizes of flanges, termed "coupling means," for placing the probe into closed communication with the interior of the pipe, and for taking into account different pipe diameters. The housing contains the remainder of the required components and electrical circuitry of the apparatus, including a battery for power, that are provided for making the pipe length determination by solving a mathematical equation.
A problem with the method and apparatus of the Kayem et al patent is a requirement for the probe to be placed in closed communication with the interior of the pipe. Additionally, the mathematical equation to be solved and the required electrical circuitry are undesirably complex since correction or compensation is required not only for the distance that the impedance reflective surface is spaced from the end of the pipe but also for the distance that the impedance reflective surface is spaced from the hand-held probe, and in particular, the means for producing sound and the means for detecting reflected acoustic waves. A further problem with an alternate method contemplated in the patent is the requirement for the operator, when making pipe length measurements, to manually insert into the computer, in each case, the diameter of the pipe being measured in order to effect compensation for the impedance reflective surface being spaced from the end of the pipe. Another alternative method and apparatus disclosed in the patent utilizes flanged tubes or coupling means having tubes of different length for taking into account the different pipe diameters. In each of the forms of apparatus that are disclosed, the requirement for a housing separate from the probe seriously detracts from the portability, and hence, the utility of the apparatus, as does also the need for the use of different flanged tubes.